This invention relates generally to a system for inspecting printed circuit boards, and more particularly, to a system for inspecting printed circuit boards with surface mounted components to verify correct component placement and proper solder connections.
As is known to persons skilled in the art, a printed circuit board is used for mounting and electrically interconnecting electrical components in a predetermined manner. Traditionally, this was accomplished using through-the-hole technology which involved the placement of components on the top side of a printed circuit board so that the connecting leads or pins of the components extended through holes (generally plated) provided in the circuit board. The leads or pins extending through the circuit board were then bent over to mechanically clamp the electrical components to the printed circuit board, followed by an appropriate soldering operation to complete the necessary connections.
It has become traditional for such printed circuit boards to be constructed mechanically, using automated assembly machines which operate to reduce the often prohibitive costs of manually assembling a printed circuit board. While reducing overall costs, such automated assembly has been found to result in periodic mis-insertions of the components, and their leads or pins, resulting in an ineffective and unreliable electrical connection. Considering the costs which inherently result from such mis-insertions, a variety of steps have been taken to locate mis-insertions, or other placement errors, as early as possible in the assembly process. This is because the cost of correcting an error increases significantly as the board proceeds through subsequent manufacturing, and distribution steps. For example, a mis-insertion which is found prior to soldering is inexpensively repaired, while a mis-insertion which is not found until final assembly (in a product) is often so costly to repair that the assembled board is often discarded rather than attempting to locate and correct the problem.
Early efforts to locate mis-insertions, and other assembly errors, involved a visual inspection of each printed circuit board at a desired stage of the manufacturing process, by human operators using the naked eye, or possibly a stereo microscope or the like. However, since it is not uncommon for a typical printed circuit board to have from 1,000 to 10,000 leads or pins for connection, such a job was found to be extremely tedious and inaccurate. Accordingly, even under the best of conditions, a significant number of missed assembly errors was found to result. Added to this were the increased costs of such an inspection process due to the significant amount of time which was required, and the correspondingly increased inventories which were necessary to accommodate such visual inspections.
For this reason, steps were taken to develop automated systems for inspecting printed circuit boards, to replace such visual inspections. One such device, which has found wide acceptance in the industry, is the Model 5511A Printed Circuit Board Inspection System which is distributed by the Universal Instruments Corporation of Binghamton, N.Y. This apparatus generally employs a series of cameras which are mounted within a fixture (an inspection head) adapted for movement upon an X-Y table to inspect various portions of a printed circuit board (from its underside) which is placed over the X-Y table. The inspection head is sequentially advanced to successive viewing fields (typically 2".times.2") established along the surface of the printed circuit board to inspect (through microprocessor analysis) the leads or pins which extend through the printed circuit board, to its bottom, to verify their effective placement based upon a comparison with pre-established norms for the particular printed circuit board which is being tested. Any defects are then reported to the operator, for appropriate correction.
The accuracy of this inspection is enhanced by providing an inspection head which incorporates a series of four angled, orthogonally placed cameras, each of which is provided with its own corresponding light source (preferably a series of controllable LED's). Such structure is provided to enable each viewing field to be inspected from four different perspectives, making sure that a lead or pin which extends from the through-hole is detected in at least one of these four orientations. Through microprocessor controls associated with the apparatus, each of a series of anticipated leads or pins can be checked for proper placement in a highly reliable and automated fashion, eliminating the need for tedious visual inspections and the like.
However, recent advances in technology have resulted in a trend toward the replacement of through-the-hole printed circuit boards with so-called surface mount technology (SMT). Rather than making use of apertures to receive leads or pins fed through the printed circuit board to its underside, for appropriate connection, SMT printed circuit boards incorporate components (both active and passive) which are placed on top of the printed circuit board so that their leads communicate with conductive pads associated with the top face of the printed circuit board. These components are then electrically connected to their associated pads by an appropriate soldering operation, from the top of the board.
With the expanded use of SMT printed circuit boards came the expanded need for automated machinery to inspect component positioning and the electrical connections which were being produced. This included the need for a variety of testing procedures, which extended beyond the basic testing procedures needed to inspect through-the-hole printed circuit boards, such as verification of the placement of SMT components, both before and after the soldering procedure, and a check on resulting soldering connections. It therefore remained to develop a system which was capable of performing these functions on an expedited basis and in automated fashion.